Gaseous-discharge device



May 2L 1940 K. J. GERMEsHAusEN 2,201,167

` GAsEoUs-DIscHARGE DEVICE 4 Filed March 9, 193? v Patented May 21, 1940without a time delay to heat the cathode.

UNITED STATES GASEOUS-DISCHARGE DEVICE Kenneth Joseph Germeshausen,Cambridge, Mass.

Application March 9, 1937, Serial No. 129,838

17 Claims.

The present invention relates to gaseous discharge devices.

An object of the present invention is to provide a new and improvedcold-cathode gaseous-discharge device of the character `described inLetters Patent 2,185,189, granted January 2,y 1940, that shall operateimmediately, without continuous consumption of cathode-heating power,and

Reference is made to the said Letters Patent for matters not more fullydisclosed herein.

Another object of the invention is to provide a new and improved gaseousdischarge device of the above-described character for controlling verylarge instantaneous amounts of power and current in response to theapplication of very small amounts of power.

A further object is to provide a new and improved grid-controlledgaseous-discharge device of the above-described character that shall beadapted to control very large surges of current in response to a verysmall current applied to its grid.

Other objects will be explained hereinafter and will be particularlypointed out in the appended claims. i

Fora consideration of what I believe to be novel and my invention,attention is directed to the accompanying description and the claimsappended thereto.

In the accompanying drawing, Fig. 1 is a longitudinal section of agaseous-discharge tube embodying the present invention; and Fig. 2 is acircuit diagram containing a tube of the abovedescribed character,showing one use to which this tube may be put.

The tube illustrated in Fig. 1 may comprise an evacuated glass envelopel, lled with a suitable gas, such as neon, or any of the other noblegases, such as argon or helium. The pressure of the gas in the tube mayvary, the usual pressure being from. one to two centimeters. The tubecontains several electrodes, namely, a solid cathode 2, an anode orplate 5, and one or more grids, inner and outer grids being shown" at 3and 4 between the anode and the cathode. The source of the electrons isa bright cathode spot on the surface of the cathode. Either grid may beused as the control grid, depending upon the polarity and the magnitudeof the control voltage.

As illustrated in Fig. l, the cathode may comprise a metal cup 33secured within and closing the lower end of a ceramic insulatingcylinder 32. In the cup is a pill 8 that may be constituted of a mixtureof materials that can react chemi- (Cl. Z50-27.5)

cally to produce a substance of relatively .low work-function, and thatdo not combine chemically with the gas in the envelope l. A pill ofcompressed caesium chloride and aluminum ii1- ings or powder answers tothis description. The 5 pill forms the active material of the cathodeand is held in place by a wire-mesh screen 9. The cylinder 32 preventsthe discharge from forming 'on the outside of the cup.

The grids may also be supported by the insulat- 10 ing cylinder 32. Tothis end, the cylinder 32 may be provided with an inwardly projectingflangeY 34, on the inner side of which is secured a metal cylinder whichserves as the grid 3. The other grid 4 comprises a carbon cylinder which15 is secured to, or rests upon, the upper end of the cylinder 32. Thecylinder 4 is placed so that caesium does not sputter on its surface.

4 As explained in the said Letters Patent, this grid 3 may, in normaluse, be placed close to zo the cathode 2, say, not more than a.f fewmillimeters from the cathode, and may be of such shape that the cathodewill sputter a thin surface layer or coating of a material of lowwork-function, such as caesium, on the grid surface. A thin 25 surfacelayer of caeslum or other material of low work-function is thusmaintained on the grid 3 during the normal operation of the tube. Afeature of the present invention is to provide a gaseous-dischargedevice of the type described and claimed in the said Letters Patent,buthaving constructional features not covered by the claims thereof.According to another feature of the present invention, however, thecoating of low work-function may be provided on the grid lby other meansthan sputtering from the cathode 2. The grid 3 may then be placed somedistance from the cathode, as it will then be no longer necessary todepend on the cathode sputtering to coat the grid 3. 40

The grid 3 may be precoated in any desired way, but a preferred methodis as follows:

Previous to assembly of the tube, the grid 3 is coated with a materialof low work-function, or that can be reduced to a material of lowworkfunction after the tube is evacuated. In the claims, both of thesewill be generically referred to under the terminology comprising amaterial of low work-function or its equivalent.

A material of this nature that has proven satlsfactory is bariumnitrate. The barium nitrate may be very finely powdered, and carried bymeans ofa suitable medium, such as collodion. After assembly, the tubeis evacuated and baked at 400 degrees centigrade for 10 to 15 minutes, u

the oven is then removed, and the tube is allowed to cool. About 1centimeter pressure of a suitable gas. such as neon, is then added tothe tube, and a discharge is passed between the grid 3 `and the grid 4.The current should 'be sufficient to heat the grid 3 to a dull red heatin about 30 seconds. This decomposes the barium nitrate to barium oxide.A discharge currentl of about 100 milliamperes at 60 cycles arternatingcurrent is suitable. As soon as the grid 3 has reached a dull red heat,the current is shut off and the tube is evacuated. The tube is thenrelledvwith the desired gas at an appropriate pressure and sealed oif.The tube is now aged by placing it in a condenser-discharge circuit suchas that illustrated in Fig. 2; after a few minutes of operation, it willbe found that the barium oxide has formed a uniform surface of lowwork-function. The surface so formed is stable in character and of longlife.

It is understood that other methods may be employed to coat the grid 3,such as the use of barium azide, which is reducible to barium, or bariumor other metals of low Work-function may be evaporated onto the grid 3from a suitable source of the metal, such as a reaction mixture placedon the grid 4 or on the anode and heated to evolve the desired material,such as barium..

The hereinafter-described cathode spot is more easily formed on the pillof caesium chloride and aluminum than on pure caesium. This is due tothe fact that the caesium chloride and aluminum are in eifect surfaceimpurities or irregularities. If the pill 8 were made of pure caesium.,furthermore, or for other reasons furnished too much free caesium, or ifthe arc discharge between the cathode and the anode werenon-intermittent, the caesium would -be quickly sputtered and evaporatedover the tube and the cathode would have very short life.

Viewed `from one aspect, therefore, the pill 8 is f composed of achemical compound of caesium which is slowly broken down under theaction of the cathode spot, liberating free caesium. Viewed from.another aspect, the pill is composed of a mixture of caesium with amaterial which retards the vaporization of the caesium under the actionof the cathode spot.

Other materials than caesium chloride and aluminum may be used for thepill 8; for example, mixtures of caesium chloride and cadmium or zinc,and mixtures of caesium chloride and rubidium chloride and aluminum,cadmium or zinc. Other metals than caesium, such as the alkali-metals,the alkali earth metals, or the rareearth metals, may be used to providethe active material of the cathode. Examples of these are mixtures ofsodium chloride and lead; mixtures of barium chloride and aluminum orzinc; barium oxide; strontium. oxide; and misch metal. All tests haveshown the caesium compounds to be most satisfactory.

In general, a chemical compound of one of the alkali, alkali-earth, orrare-earth, metals mixed with a metal which will displace the combinedmetal, or a mixture of one of the alkali, alkaliearth, or rare-earth,metals with a substance which will retard the vaporization of the metal,will be `satisfactory and will provide a cathode on which the cathodespot may be readily formed. The action of the cathode spot should evolvea material of low work-function, sothat advantage can be taken of theease of cathode-spot `formation on these materials.

The screen 9 facilitates the formation of acathode spot on the pill'byproviding a number of points for'the concentration, at the surface ofthe cathode, of the g1ow-discharge current from the anode which precedesthe arc discharge.

Irregularities and impurities on the surface of the cathode also assistin concentrating and 1ocalizing the electrical glow discharge on thecathode to produce the cathode spot on the cathode, and thereby renderthe electrical discharge an arc discharge. If the screen 9 is omitted,the local current density at the cathode may not become high enough tocause the formation of a cathode spot, and the discharge between theanode and the cathode would then hold over in a glow discharge.

The form of the arc stream between the anode and the cathode is a columnof small diameter which has high intrinsic brilliance, withsubstantially all the light emitted from the concentrated arc stream.This discharge is easily distinguished from a glow discharge, whichoccupies substantially the whole interior of the tube, with a dischargeof low intrinsic brilliance, the greatest brilliance being at thesurface of the cathode.

The grids 3 and 4 and the cylinder 32 constitute a chimney or restrictedpassage for the arc discharge, rendering the voltage drop of the arcstream more constant and, therefore, increasing the stability of theoperation of the tube.

The cylinder 32, furthermore, overcomes some tendency for the dischargebetween the anode and the cathode to become a glow discharge between theoutside of the cathode and the anode. This tendency is further resisted-by placing the active material of the cathode at the bottom of the cupor cylinder, since there is a greater tendency for a cathode spot toform on an inner surface, such as the bottom of a depression, or in acorner, than there is for the cathode spot to form on an outer surface.

The anode is so placed that caesium is not sputtered on its surfaceduring the operation of the tube. As an additional precaution, it alsomight be made of a material, such as carbon, to which the caesium doesnot readily adhere.

In the circuit of Fig. 2, a tube is illustrated having but a single grid4. Circuits embodying a two-grid tube are illustrated and described inthe said Letters Patent and in application Serial No. 129,837, filedMarch 9, 1937. The anode 5` and the cathode 2 of the tube areshownconnected across a condenser II, which is continuously charged, by wayof conducting wires 1 and 8, from. a suitable direct-current source (notshown) of, say, 300 to 400 volts, connected through a variablecurrent-limiting impedance I2 and an output element, such as a counteror other device 5I. The impedance I2 may be a resistor, or a combinationof resistance and inductance. The relay 5I is shown provided with arelay coil 5D. The wire 'I is shown as the positive conducting lead, andthe wire 8 as the `negative lead. The source may be one or morebatteries (not shown), thermionic or gaseous-discharge rectiflers (notshown) for producing direct current from alternating current, or anyother source of direct current. The grid 4 is connected to the junctionof resistors 8| and 82, thus providing a positive bias for the grid 4.

In the operation of the circuit, the condenser I I starts to becomecharged from the direct-current source, through the impedance I2 and therelay coil `5I), the tube being non-conductive. Whenthe voltage acrossthe condenser I I is equal to the voltage of the direct-current supply,the current ceases to ow. The grid bias of the tube I is adjusted sothat the voltage across the resistor II normally does not exceed thebreakdown voltage between the grid l and the rcathode 2. The tube I doesnot, therefore, normally become conducting, even when the condenser IIis fully charged. If, however, a control voltage of sumcient magnitude,from any suitable source, such as an oscillator, a contactar device,aphotocell, an amplifier, etc., is now applied between the terminals andIOI connected to the opposite ends of the resistor 8l, with the terminal|00 positive, the potential of the grid l will momentarily be raised sothat the breakdown voltage between the grid and the cathode'will then beexceeded, the tube will become conducting, and a glow discharge willconsequently take place between the grid and the cathode.

Because of this glow discharge between the grid and the cathode, anelectrical discharge will start between the anode and the cathodethrough the gas in the envelope I. If the impedance in the circuitcomprising the condenser, the anode and the cathode is low, the currentwill instantly rise to a value suicient to start a cathode spot. Thisresults in a low-drop arc discharge in the tube. The current rises to avery high value (say, 300 amperes) and the tube emits a brilliant ash oflight. The low impedance of the discharge pathpermits the condenser todischarge'almost instantly, the current and ash of light lasting a fewmicroseconds. When the voltage across the condenser has fallen to avalue approximately equal to the arc drop, the tube current ismaintained only by the flow through the charging impedance I2 and therelay coil 50. The values of this impedance and the direct currentsupply must be such that the residual current is too small to maintainthe cathode spot. When this is the case, the tube drop must rise to thatnecessary to maintain a glow discharge, which is some ten times thatnecessary to maintain the arc. By the time the voltage of the condenserhas risen to this value, the tube has had time to extinguish and returnto its normal non-conducting state and will remain so until the gridvoltage has again reached the value necessary to cause breakdown of thetube. y

The arc through the tube II is not maintained, because of the action ofthe impedance I2 with the impedance of the relay coil 50, which limitsthe current flow to such an extent that the arc is extinguished.

The fact that the condenser, together with the leads to the electrodesof the tube, forms an oscillatory circuit further assists in theextinguishing of the tube, tending to cause the voltage across the tubeto fall to a value lower than the arc drop of the tube, or even toreverse polarity.

The condenser II is thus periodically charged from the source of voltagesupply and then discharged through the tube I, from the anode to thecathode.

The impedance of the discharge circuit comprising the condenser Il andthe tube I should be sufliciently low so that the current will rise tothe value necessary to start the cathode spot (say, 5 amperes). Unlessthis is so, the circuit will not operate properly and the tube I mayhold over" into a continuous glow discharge.

The current passes between the anode and the cathode continually butinterruptedly to set up continually discharges of arc characteristicbetween the cathode and the anode. It is of amperage sufficiently highto produce a potential gradienton a relatively small area only of thecathode high enough to extract electrons from the cathode whileI thecathode remains cold, at low average temperature, and to produce a fallof cathode potential lower than that occurring in a glow discharge.I Theaverage value of the said current is nevertheless relatively low. It issufciently low, indeed, so that its root-mean-square value, that is, theheating current, through the tube, is low enough so that the averagetempera- 4ture applied by the-current to the cathode is less than thetemperature at which substantially incandescent cathode emission isproduced. The reaction takes place, therefore, at relatively lowtemperature, and is not progressive or explosive, and the cathode spotis formed on the cathode continuallyI but interruptedly. The result isthat the layer of the material of low Work-function. such as caesium, iscontinually replaced during the operation of the tube. The intervalsbetween the discharges are sufciently large so that the averagetemperature of the cathode remains low enough to prevent rapiddisintegration of the cathode. If the heating current lwere great enoughto cause the cathode to become overheated, the reaction mixture in thecathode would completely react, and the caesium would all becomedisplaced from its salt, so as to become evaporated onto the surface ofthe bulb. An approximate value of root-mean-square current for a smalltube ls 50 milliamperes.

If a glow discharge is maintained in the tube,

the permissible root-mean-square current will be less than'milliamperes, since the cathode voltage drop is higher, and hence theheating effect at the cathode will be greater for a given current.

The current required to charge the condenser Il again is used to operatethe relay or counter 5 l If the relay, counter or other electricaldevice has suflicient impedance, the resistor I2 may be omitted.

It is apparent that, for each pulse of voltage applied to the controlterminals |00 and IOI, of sufficient magnitude to start the tube I,there is a resulting pulse of current in the relay circuit. The tube Iand the control terminals |00 and IOI constitute, in elect, a relay orswitch. 'I'he magnitude and duration of the current in the relay circuitis independent of the control pulse and is determined by the supplyvoltage, the lmpedance I2, the impedance of the relay, and the condenserII. This is important in many applications, especially where theduration of the controlling-voltage pulse is extremely short,

An application of this circuit that has resulted in a very considerablereduction in apparatus and complexity over previous circuits is thecounting of cosmic rays detected by means of a Geiger counter. be placeddirectly in the grid circuit of the tube I. Although the power output ofthe counter is extremely small, and its voltage pulse of very shortduration, it is sucient to start the tube I, which then operates anappropriate mechanical counter, which may be represented by the outputelement 50.

Further modifications will occur to persons skilled in the art and allsuch are considered to fall within the scope and spirit of theinvention.

What is claimed is:

l.. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a material that will break down under the action of acathode spot Theoutput of the Geiger counter can 1 to form on thecathode a surface coating of a material of a low work-function, and thegrid comprising a material that is reducible to a material of lowwork-function.

2. A gaseous-disc/harge device comprising an anode, a cathode and a griddisposed adjacent t the cathodebetween the anode and the cathode, thecathode comprising a material that will break down under the action of acathode spot to form on the cathode a surface coating of a material of alow work-function, and the grid comprising a material of lowwork-function.

39A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a material that will break down under the action of acathode spot to form on the cathode a surface coating of a material of alow work-function, the grid comprising a material that is reducible to amaterial of low work-function, and a second grid between the first-namedgrid and the anode.

4. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a material that will break down under thev action ofa cathode spot to form on the cathode a surface coating of a material ofa low work-function, the grid comprising a material of lowwork-function, and a second grid between the first-named grid and theanode.

5. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, the'cathode comprising a material that will break down under the action of acathode spot to form on the cathode a surface coating of a material of alow work-function, and the grid having a barium coating.

6. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a material that will break down under the action of acathode spot to form on the cathode a surface coating yof a material ofa low work-function, the grid having a barium coating, and a second gridbetween the first-named grid and the anode.

7. A gaseous-discharge tube comprising an anode, an insulating cylinderprovided with an inwardly projecting flange, a cathode in the cylinderof a material which will break down under the action of a cathode spotand form a surface coating thereon of a metal of low work-function, anda cylinder held by the iiange and constituting a grid.

8. A gaseous-discharge tube comprising an anode, an insulating cylinderprovided with an inwardly projecting iiange, a cup in the cylinder, acathode in the cup of a material which will break down under the actionof a cathode spot and form a surface coating thereon of a metal of lowwork function, and a cylinder held by the iiange and constituting agrid.

9. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a `first substance and also a compound containing asecond substance of a low work-function, the compound being a materialthat will break down under the action of a cathode spot to form on thecathode a surface coating of the second substance, the iirst substancebeing a material that will replace the second substance in the compound,whereby the coating will continually break down under the action of thecathode spot to form the said surface coating on the cathode and thefirst substance will continually replace the second substance in thecompound during the continual breaking down of the compound under theaction of the cathode spot, and the grid comprising a material of lowwork-function.

10. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, aninsulating cylinder at the lower end of which the cathode is disposedand at the upper end of which the grid is disposed, the cathodecomprising a material that will break down under the action of a cathodespot to form on the cathode a surface coating of a material of a lowworkfunction, and the grid comprising a material of low work-function.

11. A gaseous-discharge device comprising an anode, a cathode and aninner grid disposed adjacent to the cathode between the anode and thecathode, the cathode comprising a material that Will break down underthe action of a cathode spot to form on the cathode a surface coating ofa material of a low work-function, the grid comprising a material thatis reducible to a material of loW-work-function, and an outer grid ofcarbon between the inner grid and the anode.

12. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a material that will break down under the action of acathode spot to form on the cathode a surface coating of a material of alow work-function, the grid comprising a material of low work-functionand cylindrical means for restricting the arc discharge.

13. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a mixture of .a caesium salt and aluminum, the gridhaving a coating comprising a material of low work-function.

14. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a mixture of caesium chloride and aluminum, the gridhaving a coating comprising a material of low work-function.

15. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent t'o the cathode between the anode and the cathode, thecathode comprising a mixture of a caesium f salt and cadmium, the gridhaving a coating comprising a. material of low work-function.

16. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathide, thecathode comprising a mixture of caesium chloride and cadmium, the gridhaving a coating comprising a material of low work-function.

17. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, thecathode comprising a compressed pill of a material that will break downunder the action of a cathode spot to form on the cathode a surfacecoating of a material of a low-work-function,

- a support for the cathode pill the grid comprising a material of lowwork-function, and a screen for holding the pill on the support and forfacilitating the formation of a cathode spot.

KENNETH J. GERMESHAUSEN.

